Some excellent points have been made in this thread, specially that co-founders can end up being employees in their own businesses, as well as it being desirable for entrepreneurs to sometimes wear an employees shoes to get the full picture (and just do a reset).
My own view is that what excites an entrepreneur is not just taking his/her own idea to fruition, but to generally accept the challenge of taking *an* idea to fruition. As a founder, it's their idea, but it could be just about anyone else's. Sense of ownership is the key, however, and it doesn't necessarily come from ownership - that's something your company will need to cultivate, and I think as long as any employee has that feeling of ownership, they will behave like a co-founder. This kind of puts your question on it's head: you would *want* him to work like a founder - in your company - and not really like an employee who watches the clock.
All the best!